And, if it is nowhere in scripture, why do you?
Now, before you go lifting scriptures out of their contexts, please understand that Acts 20 was a one-time event that occurred on what we would call a Saturday night (but their days began at sundown, so it was the first day of the week after sundown), and it had nothing to do with the day of the week believers assembled in their homes each week. It was a meal Paul had with some believers after the Sabbath had passed at sundown. It was the night before he was leaving to walk 24 miles to a ship—and, he started that walk on Sunday morning.
Also, what is stated in 1 Corinthians 16:2 has nothing to do with the day of the week believers assembled together, as it was a directive to set aside at their homes from what they produced on the first day of their work week. That’s why Paul told the Corinthians to assign representatives who would go to their houses to gather the gifts, and then journey with him to deliver those gifts to the believers in Jerusalem. If they were all in one place, then that statement would make no sense. No, the first day of the week was a work day for them, not a day of assembling together.
Those are the only places in the entire NT, outside of Christ’s actual resurrection (which took place during the night after the Sabbath had passed), that mentions anything about the first day of the week. Not only does scripture not say anything about the day of the week believers assemble together being changed to the first day of the week, the NT says nothing else about the first day of the week at all—in ANY context.
Moreover, scripture states nowhere that the day true believers are to assemble was ever to be changed because of the resurrection, for the resurrection actually occurred on a specific Sabbath—the Feast of Firstfruits, which is the first day of the barley harvest (a work day).
So, the question remains: WHY DO YOU ASSEMBLE TOGETHER ON SUNDAY?
For more information: Saturday or Sunday? Does It Even Matter?